For historical and regulatory reasons, the radio frequency spectrum is managed under two different regimes, licensed and unlicensed. In the licensed regime, a regulator such as a national authority assigns a right, generally exclusive, to an individual to operate a radio system. The assignment is typically limited in terms of the permitted time when it may be used, the permitted geographic area of operation and the permitted spectrum band. The types of limitation selected include power or EIRP within and outside the operating bandwidth, antenna height, type of modulation and so forth. The regulator chooses each assignment to prevent harmful interference to other users.
The unlicensed regime does not have exclusive assignments. Any number of users may operate any device that meets certain technical and operating restrictions in an unlicensed frequency band. The regulator sets these restrictions to minimize potential interference. Typical restrictions include:
Requirements for low transmit power or EIRP
Requirements for low duty cycle or throughput
Restrictions on mobility
Restrictions on how the devices are uses, for example, for electronic meter reading.
However, an explicit condition of unlicensed operation is that all such devices must accept any interference they receive from other unlicensed devices, even if it causes them to fail to function. Moreover, unlicensed users generally may not interfere with any licensed operation, even if they must cease transmission.
In some cases, regulators have mixed the two regimes. The so-called broadcast “white spaces” are one example that is relevant to the embodiments herein. The white spaces occur in the first place because the licensed operation, television broadcasting between 500-700 MHz, can occupy only one-third to one-half of the spectrum in any area. The rest of the spectrum was left unassigned because consumers' television receivers cannot discriminate satisfactorily between adjacent signals.
There have been proposals to use this spectrum at least since DeVany, et al. [1969] p. 1556. A recent decision by the Federal Communications Commission (FCC [2008]) has opened the white space to a form of unlicensed operation. It is the nature of unlicensed use that the FCC has placed the full burden of non-interference on the unlicensed operators. In this case, the challenge to using the white space bands is a substantial requirement to coordinate with and protect licensed users of the white space frequencies themselves or adjacent frequencies (even though use of these frequencies may be relatively minor). Specifically, so-called white space devices (WSDs) must either (a.) perform the following functions or (b.) be a client of a device that does so:
Be able to determine their position,
Consult a geolocation data base to determine which frequencies are available, and
Transmit only after they receive a “control” signal that positively identifies which frequencies are available
Notice that these requirements do not address how the unlicensed devices will avoid interfering with each other. Open access to unlicensed bands by any number of users make avoiding mutual interference a significant problem. Moreover, adopting existing techniques to avoid mutual interference leads to poor spectrum utilization. The low transmit powers, low duty cycles and other restrictions noted above necessarily reduce the overall intensity of spectrum use, measured for example as bits/sec per unit of geographic area, to a low level.
Because the service is unlicensed, it is impossible to limit the number of devices in a particular area. In general, there will be more unlicensed devices than there are channels.